linux-kernel - Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

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Date:	Wed, 4 Nov 2009 13:19:39 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Glauber Costa <glommer@...hat.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	kurt.hackel@...cle.com,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Keir Fraser <keir.fraser@...citrix.com>, zach.brown@...cle.com,
	chris.mason@...cle.com, Ingo Molnar <mingo@...hat.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall 
	implementation

On Thu, Oct 29, 2009 at 7:07 AM, Avi Kivity <avi@...hat.com> wrote:
> On 10/29/2009 04:46 PM, Dan Magenheimer wrote:
>>
>> No, the apps I'm familiar with (a DB and a JVM) need a timestamp
>> not a monotonic counter.  The timestamps must be relatively
>> accurate (e.g. we've been talking about gettimeofday generically,
>> but these apps would use clock_gettime for nsec resolution),
>> monotonically increasing, and work properly across a VM
>> migration.  The timestamps are taken up to a 100K/sec or
>> more so the apps need to ensure they are using the fastest
>> mechanism available that meets those requirements.
>>
>
> Out of interest, do you know (and can you relate) why those apps need
> 100k/sec monotonically increasing timestamps?

This is sort of tangential, but depending on the need, this might be
of interest:  Recently I've added a new clock_id,
CLOCK_MONOTONIC_COARSE (as well as CLOCK_REALTIME_COARSE), which
return a HZ granular timestamp (same granularity as filesystem
timestamps). Its very fast to access, since there's no hardware to
touch, and is accessible via vsyscall.

The idea being, if your hitting clock_gettime 100k/sec but you really
don't have the need for nsec granular timestamps, it might provide a
really nice performance boost.

Here's the commit:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=da15cfdae03351c689736f8d142618592e3cebc3

thanks
-john
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